Integrated circuits having small feature sizes, such as CMOS process nodes 90 nm and below, have many advantages in terms of cost, speed, and dynamic power consumption. However, due to the small feature size of transistors and devices in such integrated circuits, the circuits have substantial leakage currents when in sleep-mode or standby-mode.
Today's deep submicron CMOS processes allow for smaller and lower-cost ICs, providing more functionality with lower active power consumption. However, a major disadvantage with these fine-line CMOS technologies is the large standby power consumption. The increased standby power arises from various sources. Some principal contributors include: VT scaling, drain-induced barrier lowering (DIBL), direct tunneling of carriers through the gate (gate leakage), and short/narrow channel effects. Device leakage is particularly high for the minimum-sized devices commonly used in SRAM cells. Furthermore, leakage increases super-linearly with temperature, and device leakage at the top end of industrial- or military-temperature ranges can be so high as to preclude the use of fine-line CMOS.
Mixed-signal and digital products (e.g. Dust Networks DN6000, Linear Technology LTC5800), often have an “active” or run mode and a “sleep” or standby mode. During active mode, the digital circuits and possibly analog circuits are enabled and perform one or more functions. The functions may be defined by hardware (e.g., a DMA transfer) or by software (e.g., service an interrupt). To conserve power, digital devices often go into a standby mode when a function is not actively being performed. One method of reducing power during standby mode is to completely remove power from a portion of the digital circuit. Using this method, a transistor in series with the digital logic or digital memory is turned off reducing the current leakage of the switch to almost zero. While the power savings is considerable, turning off the digital blocks results in loss of state. Specifically, the contents of a static memory, as well as the state of flip-flops, are lost. For a product such as a microcontroller, a microprocessor, a memory cache, a stand alone SRAM, or a digital circuit comprising a state machine, loss of state is unacceptable.
Lower standby power consumption would be very beneficial in products such as microcontrollers or wireless sensor nodes. In addition, battery-powered products that include deep-submicron digital circuitry, such as a feature phone or a smart phone, an e-reader such as the Amazon Kindle, a tablet computer such as an ipad, or a laptop computer all would benefit greatly from reduced sleep or standby power consumption. Lower standby power translates directly into longer battery life allowing the product manufacturer to substitute a smaller, lower cost battery to attain the same lifetime between charging or, for the same sized battery, increase lifetime between charging.
A need therefore exists for circuits having small feature sizes and having leakage currents reduced by an order of magnitude or more compared to conventional circuits.